Generally, it has been known that percolating-type filters, in a large number of cases, present a very good method for the biological cleansing of waste water. They combine a reliable and service-free operation with favorable operating conditions, such as, a short contact and hold-over time, which fact results a very economical cleansing operation.
The known percolating filter-type apparatus operating with chips, as the filling material possesses, however, a considerable disadvantage in that large areas of the percolating filter body become clogged and, thereby, the effective volume of it undergoes a great variation which is beyond control, although the cloggings usually free themselves automatically. Special measures have been proposed, such as the periodical charging of the filter with relatively large amounts of waste water which was believed to aid the removal of the clogged areas. Still, the method remained uncontrollable so that there was a need for percolating-type filters having flat surface elements for the growing of the biological mass. The flat elements were introduced in the form of plates, foils or pipes. Under certain conditions, by the above-type upper surface of the flat elements, one was able to obtain a larger effective upper surface per unit volume for a percolating-type filter. However, also in the case of the above-mentioned type of percolating filters having flat surface elements, the biological mass grows for a time period until it is released as a result of biological or physical conditions prevailing on the flat surface elements. Such releasing in the above-described percolating filter apparatus occurs in small surface areas and depends on local conditions. The released biological mass then wanders together with the waste water through the percolating-type filter and is present in the waste water when it is passed through the waste output conduit. Usually, then, the sludge becomes well separated by means of sedimentation from the waste water after it is passed through a special set of clearing basins.
It is considered a disadvantage in connection with the above-described known percolating-type filters, that the waste water quantity which is necessary for the rinsing, is larger than the average quantity charged over the entire upper surface of the filter. The periodic charging of the biological mass leads locally also to a non-uniform decomposition, although the optimum decomposition is understood to be represented by a uniform charging of waste water and air.